Recently, interest in energy storage technologies has increased. As the energy storage techniques are extended to such devices as cellular phones, camcorders and notebook PCs, and further to electric vehicles, the demand for a high energy density battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and numerous studies towards improvements are now in progress actively.
Among the currently used secondary batteries, a lithium secondary battery developed in the early 1990 s includes an anode made of carbon material capable of occluding or emitting lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
The lithium secondary battery has an average discharge voltage of about 3.6V to about 3.7V, which exhibits an advantageously higher operation voltage than those of other batteries such as alkali batteries or nickel-cadmium batteries. To create such a higher operation voltage, an electrolyte composition should be electrochemically stable in a charging/discharging voltage range from 0 to 4.2V. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent for the electrolyte. The solute of the electrolyte is usually a lithium salt such as LiPF6, LiBF4 or LiClO4, which acts as a source for supplying lithium ions in the battery and thus enables the lithium battery to operate.
Lithium ions coming out from the cathode active material such as lithium metal oxide during an initial charging process of a lithium secondary battery move towards the anode active material, such as graphite, and then are intercalated between layers of the anode active material. At this time, due to the high reactivity of lithium, the electrolyte reacts with carbon of the anode active material on the surface of the anode active material such as graphite, thereby generating compounds such as Li2CO3, Li2O and LiOH. These compounds form a kind of SEI (Solid Electrolyte Interface) film on the surface of the anode active material such as graphite.
The SEI film plays the role of an ion tunnel, which allows only lithium ions to pass. Due to the ion tunnel effects, the SEI film prevents organic solvent having high molecular weight from moving together with lithium ions in the electrolyte solution and having a great molecular weight from being intercalated into layers of the anode active material and thus breaking down the anode structure. Thus, since the electrolyte solution is not contacted with the anode active material, the electrolyte solution is not decomposed, and also the amount of lithium ions in the electrolyte solution is reversibly maintained, thereby ensuring stable charging/discharging.
However, in a thin angled battery, while the above SEI film is formed, gas such as CO, CO2, CH4 and C2H6, generated by decomposition of a carbonate solvent, increases a battery thickness during a charging process. In addition, if a battery is left at a high temperature in a fully charged state, the SEI film is slowly broken down due to increased electrochemical energy and thermal energy over time. As a result, side reactions continuously occur between the exposed surface of the anode and surrounding electrolyte. Due to continuous gas generation at this time, an inner pressure of the battery is increased, thereby increasing thickness of the battery, and this may cause problems in electronics such as cellular phones and notebook computers with regard to a high-temperature performance of the battery.
In order to solve the above problems, studies have been conducted to change the phase of the SEI film forming reaction by adding an additive to a carbonate organic solvent. For example, Japanese Laid-open Patent Publication Nos. 2006-351337 and 2006-339020 disclose a non-aqueous electrolyte solution to which a fluoro group-containing sulphonate compound expressed by a predetermined chemical formula is added.
However, when the specific compound is added to an electrolyte solution to improve battery performances, some areas of performance are improved, but other areas of performance may deteriorate in many cases. Thus, there is a continuous demand to develop a non-aqueous electrolyte solution containing an additive, which may minimize such side effects.